1. Field of the Invention
The present invention is directed to a process for treating the aqueous waste from a process for the production of acrylonitrile by ammoxidation of propylene.
2. Description of the Prior Art
Typical processes for the preparation of acrylonitrile by the ammoxidation of propylene are described in U.S. Pat. Nos. 2,904,580; 3,876,508 and 3,936,360. These processes involve combining propylene, ammonia and air and passing that mixture over a suitable catalyst to produce acrylonitrile. The off gas from the reaction is initially directed to a cooling tower where the excess ammonia is neutralized with aqueous acid. Most of the desired products, e.g., acrylonitrile pass overhead through the cooling tower while the remaining products and by-products are absorbed in, or carried with, the aqueous solution exiting the base of the tower. This aqueous solution is then directed to a distillation column (waste water column) where most of the volatile materials, e.g., residual acrylonitrile and hydrogen cyanide are taken overhead. The tails from this distillation column, or waste water column, contain high boiling impurities, ammonium sulfate and reduced amounts of acrylonitrile, hydrogen cyanide and ammonia. This stream is particularly illustrated and described as the discharge through conduit 10 from column C of FIG. 1 in U.S. Pat. No. 3,876,508. Typically, this stream exhibits pH of 4.5-6.0 and contains (in percent by weight based on the total weight of the stream) 2-6% ammonium sulfate, 0.1-0.7% HCN as cyanohydrins, 0.01-0.5 acetonitrile, 0.01-.1% acrylonitrile, 0.001-.1% propionitrile, 0.001-0.05% acetaldehyde and 2.0-5.0% high boilers, e.g., nitriles such as fumaronitrile and higher molecular weight compounds such as polymers of acrylonitrile, acrolein and hydrogen cyanide. The remainder of the stream is essentially water.
The economical disposition of the aqueous waste stream from the above described acrylonitrile process has been extensively described in the art. Most of the processes are directed to the recovery of purified ammonium sulfate for use e.g., as a fertilizer. U.S. Pat. No. 3,711,597 discloses a process for the recovery of ammonium sulfate by adding nitric acid to a specific concentration and thereafter evaporating the resultant mixture at 40.degree.-120.degree. C following which ammonium sulfate is separated from the evaporated solution. U.S. Pat. No. 3,902,859 discloses the recovery of ammonium sulfate by concentrating the aqueous waste solution to the maximum degree possible while maintaining the salts in solution and thereafter adding an alcohol to precipitate the ammonium sulfate. U.S. Pat. No. 3,408,157 teaches the addition of mineral acid to the aqueous waste to precipitate heavy organics following which the material is filtered and the concentrate treated to precipitate relatively pure ammonium sulfate. U.S. Pat. No. 3,756,947 discloses a process for treating a waste water containing nitriles and cyanide by passing the waste through an activated sludge containing a specific form of bacteria. The removal of troublesome hydrogen cyanide from the waste stream by contacting with formaldehyde at a pH less than or equal to 3 is described in German Pat. No. 2,202,660. The waste stream has also been extracted with acetonitrile to remove organic matter prior to recovery of relatively pure ammonium sulfate as set forth in U.S. Pat. No. 3,607,136. Many prior techniques, for example, the process disclosed in U.S. Pat. No. 3,404,947, are concerned with disposing of the aqeuous waste stream by incineration. Alternate techniques for alleviating the substantial problem whereby this aqueous stream causes plugging of equipment when attempts are made to concentrate it involve the addition of ammonia or amines as disclosed in U.S. Pat. No. 3,468,624. More elaborate methods for treating the stream to recover ammonium sulfate are disclosed in British Patent No. 1,314,047 wherein complexing agents and a solvent such as dioxane, dimethylformamide or a lactam are added to the aqueous waste to prevent contamination of the crystallized ammonium sulfate.
None of the foregoing art discloses a process which permits the recovery of ammonium sulfate-organic containing solution which can be recovered for use as a fertilizer along with the recovery of a vapor stream containing unconsumed reactants such as ammonia which can be used elsewhere in the process and compounds such as acrylonitrile and hydrogen cyanide which can be recycled to the process.